1. Field of the Invention
This invention relates to steam cookers and blanchers for cooking and blanching a continuous or semi-continuous flow of bulk food particles. More specifically, this invention relates to an automated control process and system for such equipment.
2. Description of the Related Technology
In the conventional process of canning green vegetables such as green beans, peas and spinach, the vegetable after being washed, is subjected to a blanching operation of 3 to 5 minutes duration. The primary purpose of the blanching is to activate enzymes, such as pectin methyl esterase and expel air which would otherwise result in off-flavors. After the blanching operation, the green vegetables are filled in cans, covered with a brine solution consisting of sugar and/or salt, and the cans are thereafter sealed and sterilized.
Such methods of canning vegetables have the disadvantage that the canned product does not retain the fresh green color of the growing vegetable, but assumes a dull yellow-olive color which is unpleasant to the eye and, consequently, is not as appetizing as it might be if the fresh green color were retained. The undesirable change in color is due to hydrolytic and oxidative changes which accompany the sterilization of commercially canned green vegetables. In many cases, these degradative changes in canned vegetables are accelerated by the production of organic acids during thermal processing. Thus, the normal pH value for the fluids expressed from raw peas is about 6.6 to 7.0 whereas the product after sterilization ranges from 5.8 to 6.4. This decrease in pH constitutes an increase in acidity which causes the destruction of the natural green pigment during thermal processing and subsequent storage, i.e., the degradation of green colored chlorophyll to yellow-olive colored pheophytin.
Early processes for the retention of green color in canned green vegetables generally required an additive compound to achieve the color retention (i.e., to prevent the conversion of chlorophyll to yellow-olive colored pheophytin). These additives can be classified into two principal groups (1) alkalizing or alkalizing-buffering agents such as the hydroxides and carbonates of alkali and alkaline earth metals such as sodium, calcium and magnesium or (2) metallic salts such as the chlorides and acetates of zinc and copper.
The addition of zinc or copper salts for the retention of green color in canned vegetables has been reported in several prior art processes. For example, U.S. Pat. No. 4,473,591 to Segner et al. discloses a method having reduced processing time for the preservation of green color in vegetables. Examples are directed to treatments of green beans, green peas, and spinach, which are separately blanched in a zinc or copper ion solution for 5 to 6 minutes, after which they are packed into cans and sterilized. Blanch water metal ion concentrations of 50 to 1000 ppm are employed in this method.
In the large-scale commercial use of such systems, it is important to ensure that the aqueous concentration of metal ions is maintained within a desired process range. Conventionally, this is ensured by having the operator of the system periodically manually test the concentration of the metal ion within the blancher fluid and adjust the concentration level by adding additional metal ions and/or fluid as may be required. While this process has been used relatively effectively for many years, it is subject to human error and introduces potential quality control issues to the blanching process. A need exists for a system and method that permits a commercial blanching system to automatically monitor and adjust the composition of the blanching fluid as may be required with a minimum of human oversight and intervention.